The present invention relates to the field of powder metallurgy and in particular to the treatment of powder metal compacts.
Powder metallurgy is becoming increasingly important for producing near net shape simple- and complex-geometry components used by the automobile and appliance industries. It involves pressing metal powders to make green compacts and sintering them at high temperatures in the presence of a protective atmosphere. Small amounts of a lubricant, such as metallic stearates (zinc, lithium and calcium), ethylene bisstearamide (EBS), polyethylene waxes, etc., is usually added to metal powders prior to pressing green compacts. The addition of a lubricant reduces interparticle friction and improves powder flow, compressibility and packing density. It also helps in reducing friction between the metal powder and die wall, thereby decreasing force required to eject compacts from the die, thus reducing die wear and prolonging die life.
Although it is important to add a small amount of lubricant to metal powders prior to pressing green compacts, it is equally important to remove it from compacts prior to sintering them at high temperatures in a furnace. A continuous furnace equipped with three distinct zones: a pre-heating zone, a high heating zone, and a cooling zone is commonly used to thermally process and sinter metal powder components. The pre-heating zone of the continuous furnace is used to preheat components to a predetermined temperature. The high heating zone is obviously used to sinter components, and the cooling zone is used to cool components prior to discharging them from a continuous furnace.
It is common practice in the industry to remove the lubricant from green compacts prior to exposing them to sintering temperature in the high heating zone of a batch or continuous furnace. Improper removal of lubricant from powder metal compacts prior to sintering is known to result in poor metal bonding and produces components with low strength. It can also increase porosity, cause blistering and provide poor carbon and dimensional control in the sintered components. Furthermore, improper lubricant removal results in internal and external sooting of components and deposits in the pre-heating and high heating zones of the furnace, which in turn reduce the life of furnace components, such as the belt and muffle.
Lubricant is usually removed by (1) heating powder metal green compacts to a temperature ranging from 400.degree. F. to 1,450.degree. F., (2) melting and vaporizing the lubricant, (3) diffusing lubricant vapors from the interior to the surface of compacts, and (4) sweeping vapors away from the surface or decomposing them into smaller and more volatile components (or hydrocarbons) as soon as they diffuse out to the surface of compacts. Lubricant can be removed from compacts prior to sintering in an external lubricant removal furnace (or de-lubricating furnace) or in the preheating zone of a continuous furnace simply by sweeping vapors away from compacts with a protective atmosphere. It is believed that an effective sweeping of lubricant vapors from the surface of compacts with a protective atmosphere reduces partial pressure of vapors close to the surface of compacts, thereby (a) increasing rate of diffusion of vapors from the interior to the surface of compacts and (b) improving efficiency of removing lubricant. An effective sweeping of vapors from the surface of compacts requires very high flow rate of a protective atmosphere, making the use of high protective atmosphere flow rate economically unattractive. Furthermore, the use of a separate de-lubricating furnace is not desirable because it is expensive and it requires extra floor space which is generally not available in existing plants.
Lubricant can alternatively be removed by decomposing lubricant vapors to smaller and more volatile components as soon as they diffuse out to the surface of compacts. Decomposition of vapors to more volatile components or products as soon as they (vapors) diffuse out to the surface decreases partial pressure of lubricant vapors close to the surface of compacts, thereby accelerating the de-lubricating process. This can, once again, be accomplished in a separate de-lubricating furnace or in the pre-heating zone of a continuous furnace. For example, lubricant has been removed from compacts in a separate de-lubricating furnace by treating lubricant vapors with high temperature combustion by-products such as carbon dioxide and moisture. These separate de-lubricating furnaces are currently marketed by, Drever Company of Huntington Valley Pa., by C. I. Hayes of Cranston R. I. as a rapid burn off system (RBO), by Sinterite Furnace Division of St. Marys, Pa. as an accelerated de-lubricating system (ADS), and by Abbott Furnace Co. of St. Marys Pa. as a quick de-lubricating system (QDS). However, separate de-lubricating furnaces are expensive and require additional floor space that is generally not available in existing plants. Furthermore, they are very expensive to maintain and operate.
Decomposing lubricant vapors to smaller and more volatile components or products as soon as they diffuse out to the surface of compacts can be accomplished by using a high concentration of hydrogen in the protective atmosphere or by adding an oxidant such as air, moisture or carbon dioxide in the pre-heating zone of a continuous furnace. Numerous attempts have been made by researchers to use a high concentration of hydrogen in the protective atmosphere to decompose lubricant vapors and accelerate de-lubricating process, but with limited success. Likewise, several attempts have been made by researchers to accelerate de-lubricating in the pre-heating zone of a continuous furnace by using an oxidizing agent such as moisture, carbon dioxide or air, once again with limited success. Therefore, there is a need to develop an effective and economical method for de-lubricating powder metal compacts in the pre-heating zone (or prior to sintering them in the high heating zone) of a continuous furnace.